Removal of surface from boron carbide composite armor

ABSTRACT

The protective capability of composite armor panels is increased by removing material from the outermost layer or layers of the ceramic face plate thereof, of a thickness of from about 0.003 to about 0.050 inch, and then adhering said plate to a backing element.

United States Patent [is] 3,702,593

Fine 1 51 Nov.. 14, 1972 [541 REMOVAL OF SURFACE FROM 3,009,845 11/ 1961Wiser ..161/404 UX BORON CARBIDE COMPOSITE 3,l77,628 4/ 1965 Highberg..5 ll 2 83 ARMOR 3,573,150 3/1971 Broutman et a1. ..89/36 A [72]inventor: Richard A. Fine, Rochester, Minn. I OTHER PUBLICATIONS i 1Assignccl r n mpany, W rcester. Mass. Ceramic industry, New G-E CeramicTransmitts [22] Filed: 0c. 21, 1968 Light, pages 57 and H9, Oct. 1959.

[211 App]. No.: 772,062 Primaly Examiner-Stephen C. BentleyAttorney-Cushman, Darby & Cushman and Rufus M. Franklin [52] US. Cl..109/80, 89/36 A, 161/404 7 51 Int. Cl ..F41h 5/04 57] ABSTRACT [58]Field of Search ..23/208 A; 51/283; 89/36;

- I The protective capability of composite armor panels is 109/80161/404 increased by removing material from the outermost layer orlayers of the ceramic face plate thereof, of a I Reiemnces Citedthickness of from about 0.003 to about 0.050 inch, v UNITED STATESPATENTS and then adhering 881d plate to a backing clement.

2, 93,054 12 1954 Dietz et a1 ..89/36 A 2 'i 1 Drawing I 'i (ZZA M/d.EQCK/A f ZZiMEA/f PATENTEDIUV 14 1922 3. 7 02.59 3

JZZAM/a 1a 565 QTE INVENTOR I Xnavaez 3. l 4/5 /zzm REMOVAL OF SURFACEFROM BORON CARBIDE COMPOSITE ARMOR BACKGROUND OF THE INVENTION Newcomposite armor systems for protection against high energy projectilessuch as small arms fire, 0.30 caliber AP bullets, for example, haverecently been developed. One successful type of such armor consists of ahard, high density ceramic plate backed by a bonded laminate of glasscloth adhered thereto. When such armor is struck on its ceramic face bya projectile of sufficiently high energy, the ceramic plate shatters andfails, as does the projectile, and the energy of the projectile isdelivered to the glass cloth backing element and dissipated indelamination of the bonded layers of the glass cloth. When this occurs,persons or objects protected behind the armor suffer no injury from theimpact. Ceramic materials useful in the invention include hightemperature fabricated polycrystalline alumina, silicon carbide andboron carbide.

A useful measure of the protective capability of an armor plate or armorpanel is known as its V value.

This value is defined as the velocity at which fifty per cent ofprojectiles fired at the armor panel will penetrate it.

I have discovered that the V value can be raised several hundred feetper second, in the case of ceramic composite armor panels, merely begrinding one or both surfaces of the ceramic plates, removing materialfrom the outermost layer or layers thereof of a thickness of from about0.003 to about 0.020 inch, and up to 0.050 inch, before the plate isadhered to a backing element.

DESCRIPTION OF THE PREFERRED EMBODIMENT A standard hot-pressedmonolithic polycrystalline boron carbide plate, 0.3 l0 inch thick, isground on one side to remove approximately 0.010 inch from the surface,and then mounted by adhering to a backing element of standardconstruction. The armor thus produced exhibited a V rating of 300 feetper second greater than the armor of the same construction produced fromthe tile of the same thickness, unground, thus having the identicalareal density.

The outer layer material of the ceramic face plate may be removed bygrinding, as indicated, or may be removed as well chemically, thermallyor by EDM (electro discharge machining.)

Grinding may be with an arbor grinder, using diamond discs positionedface down. The grinding removes the outer surface material and goesquite readily until the harder underlying material is reached.Desirably, removal of the surface'layer material also improves theflatness and surface finish of the tile.

The ground ceramic tile is then adhered to a suitable backing element,such asa l2-ply glass cloth laminate bonded with a standard commercialcatalyzed polyester resin system.

THE DRAWING In the drawing, FlG. l is a sectional view of an exemplarycomposite armor panel, including a ceramic face plate 10 adhered to thefiber glass backing element 12.

The ceramic face plate, as previously indicated, is approximately 0.30inch thick, and the laminated glass fiber backing element isapproximately 0.25 inch thick.

The backing element 12 is bonded to the ceramic face plate 10 by meansof any suitable commercial resin adhesive, such as an epoxy resinadhesive. The panel, using a ground face plate, weighs approximately 6.7pounds per square foot.

It has been found that the greatest improvement in ballistic performancemay be obtained by removing outer layer material from both surfaces ofthe ceramic plate before assembly with the backing element. Thisexpedient achieves all the benefits previously suggested, and it isthought that the treated surfaces may exert some synergistic effect oneach other. While the precise mechanism of this is not known, it hasbeen suggested that a treated (and accordingly flatter and more highlyfinished) interface surface is thought to have higher tensile strengththan might otherwise be the case. High tensile strength at the backsurface of the face plate may improve the performance of the armor panelby delaying failure from the reflected shock wave following impactagainst the face surface.

The material removal step is beneficial also when only on surface isground, and the untouched surface is adhered to the backing element,leaving the treated surface exposed to ballistic impact. Conceivably,this may be because of the resultant greater uniformity of loadapplication by a projectile when it impacts. Also, the untouched exposedsurface may exhibit some sort of cushioning effect to impactingprojectiles. It is thought possible that the softer surface may fractureimpacting projectiles less completely.

This invention is generally applicable to ceramic oxide and carbidearmor plates formed by conventional hot-pressing techniques. In suchprocessing the surface of the plate, to a depth of up to 0.050 inch(depending upon the geometry of the plate and parameters of the formingprocess) is of different structure or chemistry from the body of theplate. This invention resides in the discovery that removal of thatsurface layer improves the ballistic properties of the plate when formedinto a composite armor structure. Such plates may be of mixedcomposition and include A1 0 B C, SiC, TiB and A13 Exemplarycompositions include boron carbide with additions of other carbides upto 50 percent by weight. The invention is applicable to materials formedby other fabrication techniques which produce an outer layer ofdifi'erent structure, similar to the effect produced in hot-pressing.

I claim:

' 1. Composite armor panel comprising a ceramic face plate whichcomprises boron carbide 50 percent by weight or more and a backingelement adhered thereto, the outer surface of said face plate havingbeen modified to improve its ballistic properties by removing materialfrom the outer surface layer thereof, said material ranging from about0.003 inch to about 0.050 inch in thickness.

2. Composite armor panel comprising a ceramic face plate in the form ofa hot pressed monolithic shape consisting essentially of polycrystallineboron carbide and a backing element adhered thereto, both surfaces ofsaid face plate having been modified to improve the ballistic propertiesof said panel by removing material from the outer surface layer, saidmaterial ranging from about 0.003 inch to about 0.050 inch in thickness.

2. Composite armor panel comprising a ceramic face plate in the form ofa hot pressed monolithic shape consisting essentially of polycrystallineboron carbide and a backing element adhered thereto, both surfaces ofsaid face plate having been modified to improve the ballistic propertiesof said panel by removing material from the outer surface layer, saidmaterial ranging from about 0.003 inch to about 0.050 inch in thickness.